Random Particle Image Textures
This setup is a bit fiddly but it gets the job done :-P





The particle system needs to be set to render a UV unwrapped object, in this case it's a simple plane.

(The billboard particle sadly doesn't work in cycles 2.79b. In Blender Internal there is already functionality to randomise with tiles in the billboard particle settings.)



I am using the object random input in the material to get a normalised random scalar (0 - 1). I can then use this to scale and offset the uvmap to select one of 4 images in a texture.

The input value (2) is the number of horizontal and vertical tiles and can be changed if needed.